Method and device for producing compressed nitrogen

ABSTRACT

A method and a device that serve for producing compressed nitrogen by low-temperature decomposition of air in a distillation column system which has a high-pressure column, a low-pressure column, a high-pressure column head condenser and a low-pressure column head condenser. A main air compressor constitutes the only gas compressor powered by external energy. An internally compressed nitrogen stream is formed by a part-stream of the liquid nitrogen stream from the high-pressure column head condenser and/or a part-stream of the liquid nitrogen stream from the low-pressure column head condenser; the internally compressed nitrogen stream is brought in the liquid state to a product pressure which is between 15 and 100 bars; then the internally compressed nitrogen stream is heated in the main heat exchanger and then extracted as a gaseous compressed nitrogen product below the product pressure.

The invention relates to a method of producing compressed nitrogen according to the preamble of claim 1.

A process of the type specified at the outset is known from U.S. Pat. No. 6,141,989. Here, in a distillation column system having three or even four condenser-evaporators, compressed nitrogen is produced under a product pressure of 12 bar by internal compression. This involves bringing nitrogen in liquid form, to the product pressure and then evaporating it in indirect heat exchange with air and warming it to about ambient temperature.

The pressure figures here generally do not include the natural pressure drops. Pressures here are regarded as “equal” when the pressure difference between appropriate points is not greater than the natural conduction losses which are caused by pressure drops in pipelines, heat exchangers; coolers, adsorbers, etc. For example, the internal compression nitrogen stream experiences a pressure drop in the passages of the main heat exchanger; nevertheless, the release pressure of the compressed nitrogen product downstream of the main heat exchanger and the pressure upstream of the main heat exchanger are referred to here equally as “the product pressure”.

A “condenser-evaporator” refers so a heat exchanger in which a first condensing fluid scream enters into indirect heat exchange with a second evaporating fluid stream, every condenser-evaporator has a liquefaction space and an evaporation space consisting respectively of liquefaction passages and evaporation passages. In the liquefaction space the condensation (liquefaction) of a first fluid stream is conducted, and in the evaporation space the evaporation of a second fluid stream. The evaporation and liquefaction spaces are formed by groups of passages in a heat-exchanging relationship with one another.

The “main heat exchanger” serves to cool feed air in indirect heat exchange with return streams from the distillation column system, it may be formed from a single heat exchanger section or a plurality of heat exchanger sections connected in parallel and/or in series, for example from one or more plate heat exchanger blocks. The main heat exchanger in this context is formed, for example, in U.S. 6,141,989 by the combination of a gas-gas exchanger with a condenser-evaporator in which pumped high-pressure column nitrogen is evaporated in indirect heat exchange with a condensing substream of the feed air. In the main heat exchanger of the invention, the “internal compression nitrogen stream” is evaporated against a high-pressure air stream (or pseudo-evaporated if its pressure is supercritical) and warmed. At the same time, the high-pressure air is cooled and liquefied or, if its pressure is supercritical, pseudo-liquefied. In the invention, preferably no separate condenser-evaporator is used for nitrogen evaporation; instead, the (pseudo-) evaporation and the warming take place in an integrated main heat exchanger.

in “oxygen-enriched product gas stream” is understood here to mean any gaseous product stream or residual stream which is released by the system and has an oxygen content higher than that of air. This may be very pure oxygen or else an only slightly oxygen-enriched residual gas. The process of the invention may have one or more streams of this kind.

Since the compressed nitrogen product of the abovementioned U.S. Pat. No. 6,141,989 is produced under pressure of 12 bar, for many applications, it has to be compressed farther with the aid of a gas compressor outside the air fractionation plant, for example to 100 bar.

The known process thus requires two externally driven compressors 3 and 5, as shown in schematic form in FIG. 1, in order to provide a compressed nitrogen product under more than 12 bar. Especially when there is no high-performance grid for electrical power at the site of the plant and a separate drive unit such as a gas turbine unit 1 is available, this means several steps for transmission of the mechanical energy generated at the gas turbine shaft to the drive shafts of the compressors 3 and 5: first of all, the mechanical energy is converted to electrical energy in a generator 8 and brought to a suitable voltage in a transformer 2. Subsequently, the electrical energy is converted back to mechanical energy in the motors 4 and 6. Air fractionation in the narrower sense 7 (ASU) is represented by a conventional air fractionation plant which produces mid-pressure nitrogen (PGAN—pressurized gaseous nitrogen) under a pressure of, for example, 12 bar. This mid-pressure nitrogen then has to be compressed further in a nitrogen gas compressor 5 to a pressure of, for example, 100 bar in order to be able to release it as high-pressure nitrogen (HPGAN—high pressure gaseous nitrogen), for example to promote mineral oil production (to EOR—enhanced oil recovery). (The way in which the gas turbine unit 1 works is elucidated, in detail below for FIG. 2.)

It is an object of the invention to produce a compressed nitrogen product under a very high pressure in an energetically particularly favorable manner and at the same time to use an apparatus of comparatively low complexity.

This object is achieved by the totality of the features of claim 1.

In the invention, a compressed nitrogen product under a pressure of 15 to 100 bar is obtained directly by internal compression. If this pressure is sufficient for the specified application (for example in mineral oil production—EOR=enhanced oil recovery), the caseous compressed nitrogen need not be recompressed outside the air fractionation plant and the corresponding energy expenditure and apparatus complexity is dispensed with. Of course, external further compression beyond the internal compression pressure is also possible in the invention; but even in that case, the energy expenditure is relatively low and a relatively small nitrogen gas compressor is sufficient.

The operating pressures of the columns (at the top in each case) in the invention are

6 to 12 bar in the high-pressure column 202 and

25-3 to 5 bar in the low-pressure column 204.

In principle, of it is known that nitrogen internal compressions can be conducted within the pressure range from 15 to 100 bar. On application of this known teaching to the process of U.S. Pat. No. 6,141,198, the person skilled in the art, however, would compress only the portion of the air absolutely necessary for the internal compression to an elevated pressure more than 5 bar above the high-pressure column pressure. Compression of the overall air to this very high pressure would appear to be an avoidable waste of energy, because this high pressure appears to be of barely any benefit to the remaining portion of the teed air not required for the internal compression.

In the context of this invention, however, it has been found that, surprisingly, in this specific process which serves principally or exclusively to produce compressed nitrogen by internal compression, specifically in the case of compression of the overall air to a very high pressure, a method which is particularly favorable in terms of energy is the overall result.

At the same time, the apparatus complexity can also be kept comparatively low by using only a single externally driven compressor in spite of the high air pressure, namely the main air compressor. This of course does not rule out single-stage turbine-driven compressors (boosters) which do not require any external energy but are effectively driven by the energy generated in the main air compressor, which is converted to mechanical energy in the work-performing decompression in the turbine.

Preferably, the “first substream” which is decompressed to perform work is formed by the overall remainder of the overall air stream which is not required as “second substream” for the internal compression. The “first substream” of the air can be cooled before the work-performing decompress ion to below ambient temperature, especially in she main heat exchanger to an intermediate temperature between the temperatures of the warm and cold ends of the main heat exchanger. It then enters the work-performing decompression in the caseous state and is ultimately introduced at least partly in the gaseous state into the distillation column system, especially into the high-pressure column. The gas content of the first substream forms the ascending vapor in the lower region of the high-pressure column. As an alternative to gaseous introduction into the high-pressure column, the first substream decompressed to perform work can be partly or fully liquefied in a reboiler of the high-pressure column and fed in liquid form into the high-pressure column; the ascending gas in the lower region of the high-pressure column is then formed by the vapor generated in the high-pressure column reboiler.

Further energy can be saved in the invention through a specific configuration of the distillation column system as described, in claim 2. The distillation column system, is thus not operated like a conventional Linde double column (in other words, the high-pressure column top condenser is not cooled by bottoms liquid from the low-pressure column); instead, the high-pressure column top condenser is operated with exclusively with liquid air (i.e. with a liquid having the same or a similar composition to atmospheric air). The “second portion of the feed air” is introduced, directly or via a separator (phase separator) which may be disposed in a separate vessel, or incorporated into the high-pressure column, into the evaporation space of the high-pressure column top condenser, without previously being involved in the rectification in any of the columns in the distillation column system.

Especially when this liquid air is the sole cooling fluid for the nigh-pressure column top condenser, the two columns may be arranged alongside one another without any need for process pumps for raising liquids. This makes the system more compact and allows the columns to be substantially prefabricated and then transported to the erection site.

Preferably, the maim air compressor has a single drive unit which is formed especially by a gas turbine unit, a steam turbine, a gas engine or a diesel engine. In that case, this drive unit is the sole source of external energy in the entire system apart from liquid pumps which consume very much less energy than gas compressors, and apart from the energy supply for auxiliary devices such as regulation and control units, lighting, etc. This achieves a very substantial simplification of the compressor drive; generators, transformers and electric motors for the gas compression are unnecessary and hence cannot contribute to energy losses. This is especially true when the main air compressor is the sole external energy-driven gas compressor which is used in the method.

In addition, it is favorable when, in the method of the invention, the second substream of the high-pressure overall air stream is decompressed and hence liquefied in a decompression turbine (fluid turbine) rather than being decompressed in a throttle valve, and is then introduced into the distillation column system.

It is possible to dispense with a reboiler in the low-pressure column by introducing vapor generated in the evaporation space of the high-pressure column top condenser into the bottom region of the low-pressure column, this vapor forming the entirety of the gas ascending in the lower section of the low-pressure column.

In a first configuration of the invention, a “third substream” of the first liquid nitrogen stream is applied as reflux to the low-pressure column, and the internal compression nitrogen stream is formed by a second substream of the second liquid nitrogen stream. Through the use of liquid nitrogen from the high-pressure column top condenser as reflux for the low-pressure column, it is possible to feed a corresponding elevated proportion of liquid nitrogen from the low-pressure column top condenser to the internal compression. (The expression “third substream” means here that a “second substream” of the first liquid nitrogen stream may but need not exist in the method.)

In a second configuration of the invention, the internal compression nitrogen stream is formed by a second substream of the first liquid nitrogen stream (from the high-pressure column top condenser) and a second substream of the second liquid nitrogen stream (from the low-pressure column top condenser), these two substreams being brought to the product pressure separately. When nitrogen is being consumed under two or more different product pressures, it is also possible in a modified embodiment to bring the two second substreams to different product pressures or to decompress substreams to the pressures required (after pumping); the different internal compression nitrogen streams under the different pressures are then conducted separately through the main heat exchanger and obtained as compressed nitrogen products under different pressures.

In a third configuration of the invention, the second substream of the second liquid nitrogen stream (from the low-pressure column top condenser) is first brought to about high-pressure column pressure in the liquid state, and then combined with the second substream of the first liquid nitrogen stream (from the high-pressure column top condenser) to the. The mixture then constitutes the internal compression nitrogen stream and is brought collectively from the high-pressure column pressure to the product pressure in a further step.

Cooling energy in the method of the invention is preferably generated in a single decompression machine, with at least partial introduction of the first substream of the high-pressure overall air stream, downstream of the work-performing decompression thereof, into the high-pressure column. The decompression machine may be formed, for example, by an expansion turbine. It may be coupled to a recompressor in which the first substream of the high-pressure overall air stream downstream of the work-performing decompression thereof or the second substream of the high-pressure overall air stream or the high-pressure overall air stream is recompressed to a pressure higher than the overall air pressure.

In principle, the method of the invention needs just two condenser-evaporators: the high-pressure column top condenser and the low-pressure column top condenser. In specific cases, however, it may be favorable to use a third condenser evaporator in the form of a high-pressure column reboiler. Bottoms liquid from the high-pressure column is evaporated therein in indirect heat exchange with condensing air which is introduced in the form of a third portion of the high-pressure overall air stream into the liquefaction space of the high-pressure column reboiler. The evaporated bottoms liquid is introduced into the high-pressure column as ascending gas and enhances the separating action therein. The “third portion” of the high-pressure overall air stream can be formed by the first substream which has been decompressed to perform work or by a portion thereof.

In all variants of the method of the invention, energy can be saved by recompressing the second substream of the air in a turbine-driven recompressor to a second air pressure higher than the overall air pressure. The recompressor (booster) is preferably driven by the decompression machine in which the first substream of the air is decompressed so perform work.

It is also favorable when the second substream (52) of the high-pressure overall air stream (11, 811) is decompressed and hence liquefied in a decompression turbine (rather than the decompression in a throttle valve) and then introduced into the distillation column system. This decompression turbine (fluid turbine) is preferably attenuated by a generator which generates electrical energy.

The energy efficiency in the method of the invention can be further improved by a booster circuit for the high-pressure column (claim 12) or a booster circuit for the low-pressure column (claim 13) or by a combination of these two booster circuits.

The invention also relates to an apparatus according to claim 14. The apparatus of the invention can be supplemented by apparatus features corresponding to the features of the dependent method claims.

The invention and further details of the invention are elucidated in detail hereinafter with reference to the working examples shown in schematic form in FIGS. 2 to 12. These show:

FIG. 2 the operation of an air fractionation plant of the invention with a gas turbine unit as the sole drive,

FIGS. 3 to 5 three embodiments of the distillation column system of a system of the invention,

FIGS. 6 and 7 two embodiments of a cooling and liquefaction unit of a system of the invention and

FIGS. 8 and 9 two embodiments of air pretreatment and cooling and liquefaction units of a system of the invention,

FIG. 10 one working example of the method of the invention in overview.

FIGS. 11 and 12 two further working examples of the invention with booster circuits.

FIG. 2 shows part of the air fractionation plant downstream of the main air compressor 9, merely in schematic form as box 10(ASU). The interior of this box conceals an air pretreatment unit, a cooling and liquefaction unit and a distillation column system. These process steps are presented in detail in the drawings which follow.

In the main air compressor 9 having several stages with intermediate cooling, atmospheric air (AIR) is compressed to an overall air pressure which is higher than 20 bar and in a specific numerical example is 37.5 bar. The high-pressure overall air stream 11 (HP-AIR) which exits the main air compressor 9 is introduced into the air fractionation plant in the narrower sense 10. Thence, an internally compressed nitrogen product stream 12 (ICGAN—internally compressed gaseous nitrogen) is drawn off under a product pressure above 60 bar, for example at about 70 bar. The nitrogen product stream 12 is used in the use example to promote mineral oil production (to EOR—enhanced oil recovery).

Ail stages of the main air compressor are driven by means of a common, shaft connected to the shaft of a gas turbine unit 1 which a gas turbine compressor 13, a gas turbine combustion chamber 14, a gas turbine expander 15 and—optionally—a steam raising unit 16 (HRSG—heat recovery steam generation). The gas turbine compressor 13 compresses ambient air (amb); the combustion chamber 14 burns natural gas (NG) with the compressed air. Heat is withdrawn from the combustion gas from the combustion chamber 14 in the steam-raising; the cold combustion gas—optionally after cleaning—is blown back into the ambient environment (amb).

FIG. 3 shows a first working example of a distillation column system as used in the invention. The distillation column system has a high-pressure column 202 with high-pressure column top condenser 204 and a low-pressure column 203 with low-pressure column top condenser 205. Both top condensers take the form of condenser-evaporators. The operating pressures of the columns (at the top in each case) in the example are

6 bar in the high-pressure column 202 and

3.5 bar in the low-pressure column 203.

A “first substream” 201 (AIR) of the high-pressure overall air stream 11 (see FIG. 2), after decompression to perform work in the cooling and liquefaction unit (see FIGS. 6 to 9), is introduced under a pressure of 6 bar in gaseous form into the lower region of the high-pressure column 202. A “second substream” 206 (JT-AIP=Joule-Thomson air) of the high-pressure overall air stream is withdrawn under a pressure of 3.5 bar from the cooling and liquefaction unit, decompressed to high-pressure column pressure in a throttle valve 207 and introduced via line 203 in the predominantly liquid state into a cup 209 disposed within the high-pressure column 202 at an intermediate point. This cup 209 serves as separator (phase separator).

The gaseous component from line 208 ascends within the high-pressure column 202; the liquid component is at least partly withdrawn again and introduced via line 210 and throttle valve 211 into the evaporation space of the high-pressure column top condenser 204, as is a further liquid air stream 221 which is formed by the small amount of liquid generated in the work-performing decompression. Vapor generated in the evaporation space of the high-pressure column top condenser 204 is drawn off via line 212 and fed to the lower region of the low-pressure column 203 as ascending vapor. Gaseous top nitrogen 213 from the high-pressure column 202 is introduced at least in a first portion 214 into the liquefaction space of the high-pressure column top condenser 204 and liquefied at least partly therein, preferably completely or almost completely. This forms a “first liquid nitrogen stream” 215. A first substream 216 of the first liquid nitrogen stream 215 is applied as reflux liquid to the high-pressure column 202. Another portion (the “third substream”) 217 of the first liquid nitrogen stream 215 is cooled in a subcooling countercurrent heat exchanger 218 and, after throttle expansion 219, fed via line 220 into the low-pressure column 203. (In the working example of FIG. 3, there is no “second substream” of the first liquid nitrogen stream 215.)

The oxygen-enriched bottoms liquid 222 from the high-pressure column 202 is subcooled in the subcooling countercurrent heat exchanger 218 and introduced via throttle valve 223 and line 224 into the evaporation space of the low-pressure column top condenser 205. A further cooling fluid for the low-pressure column top condenser 205 is formed by the cooling liquid 225 of the low-pressure column 203 which, is likewise subcooled in the subcooling countercurrent heat exchanger 218, throttled (226) and introduced (227) into the evaporation space of the low-pressure column top condenser 205. In addition, purge liquid 228 from the high-pressure column top condenser 204 is introduced into the evaporation space of the low-pressure column top condenser 205. A purge liquid is likewise drawn off from the evaporation space of the low-pressure column top condenser 205 and discarded or compressed to the supercritical pressure and passed through the main heat exchanger.

In the liquefaction space of the low-pressure column top condenser 205, at least a first portion 231 of the gaseous top nitrogen 230 from the low-pressure column 203 is liquefied at least partly, preferably completely or almost completely. This forms a second liquid nitrogen stream 232. A first substream 233 of the second liquid nitrogen stream 232 is applied as further reflux liquid to the low-pressure column 203. A second substream 234 is fed to an internal compression and brought therein in a pump 235 in the liquid state to a product pressure which is between 20 and 100 bar and in the example is about 70 bar. The supercritical nitrogen (ICLIN—internally compressed liquid nitrogen) is fed via line 236 to the cooling and liquefaction unit.

Vapor formed in the low-pressure column top condenser 205 is drawn off via line 240, warmed in the subcooling countercurrent heat exchanger 218 and finally fed as residual gas (WASTE) to the cooling and liquefaction unit.

Via line 237 or 238, a portion of the gaseous top nitrogen from the high-pressure column 202 or the low-pressure column 203 can be obtained directly as gaseous pressure product (HPGAN—high pressure gaseous nitrogen/MPGAH—medium pressure gaseous nitrogen), which is of course warmed up to about ambient temperature in the cooling and liquefaction unit. If required, it is also possible for a third substream 239 of the second liquid nitrogen stream 232 to be obtained as liquid nitrogen product (LIN—liquid nitrogen).

As an alternative to the embodiment of FIG. 3 with cup 209 within the high-pressure column 202, a separate separator (phase separator) can be used; in that case, the gaseous component is introduced into the high-pressure column, and the liquid component at least partly into the evaporation space of the high-pressure column top condenser 204.

While only a single internal compression pump 235 is used in FIG. 3, two of them are used in FIG. 4. A first pump 335 a brings the second substream 334 of the second liquid nitrogen stream 232, as in FIG. 3, from the low-pressure column pressure to the product pressure; in contrast to FIG. 3, however, this stream comprises not the entire internally compressed product but only a portion. The remainder of the internally compressed, nitrogen is formed by a second substream 319 of the first liquid nitrogen stream 215, which is brought from the high-pressure column pressure to the product pressure in a second pump 335 b. The two internal compression nitrogen streams are combined at 337 and together form the supercritical nitrogen (ICLIN) in line 336.

In addition, in FIG. 4, a portion 343 of the liquid air from the cup 209 is cooled in the subcooling counter current heat exchanger 318 and fed via line 344 to the low-pressure column 203 at an intermediate point.

FIG. 5 differs from FIG. 4 by a third condenser-evaporator: the high-pressure column reboiler 438. In the evaporation space thereof, the gaseous air 401 (AIR) is fully condensed. The liquid 442 formed is additionally introduced into the cup 209.

FIGS. 5 and 7 show two embodiments of a cooling and liquefaction unit 50 which can be combined with FIG. 2 and any of the distillation column systems of FIGS. 3 to 6.

In FIG. 6, the entire cleaned high-pressure overall air stream 811 (see FIGS. 8 and 9) is fed under the overall air pressure to the warm end of a main heat exchanger 51 implemented here as a single plate heat exchanger block.

A first substream 56 of the high-pressure overall air stream 811 is withdrawn from the main heat exchanger at a first intermediate temperature and fed to an expansion turbine 57 which drives a recompressor 53. The air 58 decompressed to perform work is introduced into a separator (phase separator) 59. The majority of the first substream 58 decompressed to perform work is introduced via line 201 in gaseous form into the high-pressure column of the distillation column system; the liquid 221 removed is treated as described in FIG. 3.

A second substream 52 of the high-pressure overall air stream 811 is withdrawn again at a second, higher intermediate temperature and recompressed to about 50 bar in the turbine-driven recompressor (booster) 53 with recooler 54. The recompressed second substream 55 is introduced back into the warm end of the main heat exchanger 51, conducted down to the cold end therein and pseudo-liquefied therein. The supercritical second substream 206 is introduced into the distillation column system in the manner shown in FIGS. 3 to 6.

The supercritical internal compression nitrogen 236/336 (ICLIN) flows under the product pressure to the cold end of the main heat exchanger 31. It is pseudo-evaporated therein and warmed to an out ambient temperature and finally obtained in gaseous form as internally compressed nitrogen product 60 (ICGAN). The gaseous streams 237, 238 and 241 from the distillation column system are also warmed to about ambient temperature in the main heat exchanger 51. The warmed nitrogen streams are released as products 61/62 (HPGAN/MPGAN). The warmed residual gas stream is partly blown off into the ambient environment (amb) via line 63 and partly used as regeneration gas in the cleaning unit 802 for the feed air (see FIGS. 8 and 9).

In the distillation column systems of FIGS. 3 to 5, the high-pressure column 202 and low-pressure column 203 arranged alongside one another. If a particularly small footprint is available, however, it is also possible in the invention to arrange the columns one on top of another similarly to a conventional Linde double column. In this case, beginning from the bottom, the following apparatus parts are positioned in a line one on top of another:

high-pressure column 202 (in the case of FIG. 5 with a high-pressure column reboiler 438 incorporated into the high-pressure column)

high-pressure column top condenser 204

low-pressure column 203

low-pressure column top condenser 205.

FIG. 7 differs by the line 765 from FIG. 6. The second substream 752 of the high-pressure overall air stream is not precooled here in the main heat exchanger but by the admixing of a small portion 765 of the cold first substream 756 before entry into the turbine 57.

FIG. 8 shows, as well as the cooling and liquefaction unit 50 from FIG. 6, a first embodiment of an air pretreatment unit 799. The compressor stages 804, 806 and 807 and the coolers 805 and 808 of the main air compressor 9 are not part of the air pretreatment unit.

The compressed overall air 800 is cooled to about ambient temperature in a pre coo ling unit 801 and then cleaned in a cleaning unit 802 having molecular sieve adsorbers. The cleaned high-pressure overall air stream 811 compressed to the overall air pressure is introduced into the cooling and liquefaction unit 50.

In principle, the air pretreatment unit 799 can be operated under the overall air pressure (see FIG. 10). Given the high air pressures used here, however, it is more favorable in most cases to operate the air pretreatment unit as shown in FIG. 8 under a lower pressure, by arranging it between two stages 806 and 807 of the main air compressor. This makes the design of molecular sieve adsorber vessels and switching valves simpler and lowers the losses on changeover of the adsorbers.

All three stages 804, 806 and 807 of the main air compressor 9 of the working example are driven by a single shaft connected to the shaft of a gas turbine unit, a gas engine or another engine. Between the first two stages 804 and 806 is disposed an intermediate cooler 805, and beyond the third and last stage a

FIG. 9 differs from FIG. 8 in that a portion 902 of the supercritical high-pressure air 901 is not conducted via line 206 to the distillation column system but decompressed in a throttle valve 903 to an intermediate pressure corresponding to the inlet pressure of the last stage 807 of the main air compressor 9 (plus conduction losses). It is warmed fully in the main heat exchanger 51 and then fed to the inlet of the last stage 807 of the main air compressor 9. This increases the amount of high-pressure air used for evaporation of internal compression nitrogen. This leads to a reduction in the final pressure of the main air compressor, and it is possible to reduce the number of compressor stages (cost advantage).

FIG. 10 shows a working example of the method of the invention in its entirety, here, the precooling unit 801 and the cleaning unit 802 of the air pretreatment unit 799 are operated under the overall air pressure. The main air compressor 9 has multiple stages with intermediate cooling and is formed by a single machine. The shaft thereof is driven by an electric motor, si steam turbine, a gas turbine unit, a gas engine or another engine, for example a diesel engine.

The cooling and liquefaction unit 50 shown in FIG. 10 corresponds to that of FIG. 6, and the distillation column system 1000 to that of FIG. 3.

The air pretreatment unit 799 and the main air compressor 9 of FIG. 10 can be combined with any of the other cooling and liquefaction units described here and with any of the other distillation column systems described in this application.

The working example of FIG. 11 is based on FIG. 10. It has a fluid turbine (dense fluid turbine) 1107 rather than the throttle valve 207 and is also equipped with a booster circuit.

In the fluid turbine 1107, it is possible to recover a considerable portion of the pressure energy present in the second substream 206 and convert it to electrical energy in a generator 1171 connected to the turbine. This measure makes a perceptible contribution to the favorable energy balance of the method of the invention and can also be used in all the working examples described above. Conversely, the working examples of FIGS. 11 and 12 can also be operated with a throttle valve rather than the fluid turbine.

Vapor drawn off from the evaporation apace of the high-pressure column top condenser 204 is fed here only in a portion 1112 to the low-pressure column 203. The remainder forms a circulation stream 1172 which is compressed in a one-stage compressor (booster) 1173, which takes the form of a cold compressor, from about 3.9 bar to about 6.9 bar. The compressed circulation stream 1174 is introduced into the main heat exchanger 51 at an intermediate point and cooled therein up to the cold end. The cold circulation stream 1175 is fed back to the high-pressure column 202 at the bottom and boosts the separation process therein. Therefore, the whole circuit is referred, to as a booster circuit.

The work-performing decompression of the first substream 56 of the high-pressure overall air stream 11 is conducted in two parallel-connected expansion turbines 57 a, 57 b. The first turbine 57 a, as before, drives the warm recompressor 53 for the first substream 752, and the second turbine 57 b the cold compressor 1173. The two turbines 57 a, 57 b in the working example have the same inlet, and outlet parameters in terms of pressure and temperature (in other working examples, the inlet temperatures may also be different).

FIG. 12 differs from FIG. 11 in that the booster circuit 1272/1273/1274/1275 leads not from the evaporation space of the high-pressure column top condenser into the high-pressure column but from the top of the low-pressure column 203 into the liquefaction space of the high-pressure column top condenser 204. This increases the conversion in the high-pressure column top condenser 204 and hence the amount of ascending gas 212 for the low-pressure column 203, i.e. the conversion in the low-pressure column.

Similarly to FIG. 3 (HPGAN, MPGAN), it is also possible in the methods of FIGS. 11 and 12 to additionally draw off gaseous nitrogen product directly from the top of the high-pressure column 202 and/or of the low-pressure column 203 and warm it in the main heat exchanger 51.

The booster circuits shown in FIG. 11 and FIG. 12 can be combined with any of the other cooling and liquefaction units described here and with any of the other distillation column systems described in this application. 

1. A method of producing compressed nitrogen by low-temperature fractionation of air in a distillation column system having a high-pressure column and a low-pressure column and a high-pressure column top condenser and a low-pressure column top condenser, both in the form of condenser-evaporators, wherein the low-pressure column has the lowest operating pressure of all the separating columns in the distillation column system, feed air is compressed in a main air compressor to an overall air pressure, forming a high-pressure overall air stream, the main air compressor is the only external energy-driven gas compressor which is used in the process, a first substream of the high-pressure overall air stream is decompressed to perform work and introduced into the distillation column system, a second substream of the high-pressure overall air stream is cooled down in a main heat exchanger and introduced at least partly in liquid form into the distillation column system, gaseous top nitrogen from the high-pressure column is at least partly liquefied in the high-pressure column top condenser, forming a first liquid nitrogen stream, a first substream of the first liquid nitrogen stream is applied as reflux liquid to the high-pressure column, gaseous top nitrogen from the low-pressure column is at least partly liquefied in the low-pressure column top condenser, forming a second liquid nitrogen stream, a first substream of the second liquid nitrogen stream is applied as reflux liquid to the low-pressure column, an internal compression nitrogen stream which is formed by a second substream of the first liquid nitrogen stream and/or a second substream of the second liquid nitrogen stream, in the liquid state, is brought to a product pressure higher than the operating pressure of the high-pressure column, the internal compression nitrogen stream which has been brought to the product pressure is warmed in the main heat exchanger and then drawn off as gaseous compressed nitrogen product stream under the product pressure, at least one oxygen-enriched product gas stream is drawn off from the distillation column system and warmed in the main heat exchanger and all oxygen-enriched product gas streams are drawn off from the distillation column system in the gaseous state, characterized in that the exit pressure of the first substream of the high-pressure overall air stream in the work-performing decompression is equal to or higher than the operating pressure of the high-pressure column, the feed air is compressed in the main air compressor to an overall air pressure at least 5 bar above the operating pressure of the high-pressure column, and the internal compression nitrogen stream in the liquid state is brought to a product pressure between 15 and 100 bar.
 2. The method as claimed in claim 1, characterized in that at least a portion of the second portion of the feed air is introduced as cooling fluid into the evaporation space of the high-pressure column top condenser, with formation of the entirety of the cooling fluid for the high-pressure column top condenser by feed air.
 3. The method as claimed in claim 1, characterized in that the main air compressor has a single drive unit which is especially formed by a gas turbine unit, a steam turbine, a gas engine or a diesel engine.
 4. The method as claimed in claim 1, characterized in that at least a portion of the second substream of the high-pressure overall air stream, after it has been cooled down in the main heat exchanger, is decompressed to perform work in a fluid turbine.
 5. The method as claimed in claim 1, characterized in that vapor generated in the evaporation space of the high-pressure column top condenser is introduced into the bottom region of the low-pressure column, this vapor forming the entirety of the gas ascending in the lower section of the low-pressure column.
 6. The method as claimed in claim 1, characterized in that a third substream of the first liquid nitrogen stream is applied as reflux to the low-pressure column and the internal compression nitrogen stream is formed by a second substream of the second liquid nitrogen stream.
 7. The method as claimed in claim 1, characterized in that the internal compression nitrogen stream is formed by a second substream of the first liquid nitrogen stream and a second substream of the second liquid nitrogen stream, these two substreams being brought separately to the product pressure.
 8. The method as claimed in claim 1, characterized in that the first substream of the high-pressure overall air stream, downstream of the work-performing decompression thereof, is at least partly introduced into the high-pressure column.
 9. The method as claimed in claim 1, characterized in that the bottoms liquid of the high-pressure column is evaporated in a high-pressure column reboiler which takes the form of a condenser-evaporator, with a third portion of the high-pressure overall air stream which is formed at least partly by at least a portion of the first substream which has been decompressed to perform work being introduced into the liquefaction space of the high-pressure column reboiler.
 10. The method as claimed in claim 1, characterized in that the second substream of the high-pressure overall air stream is recompressed in a turbine-driven recompressor to a second air pressure higher than the overall air pressure.
 11. The method as claimed in claim 1, characterized in that at least a portion of the second substream of the high-pressure overall air stream is decompressed in a decompression turbine and hence liquefied and then introduced into the distillation column system.
 12. The method as claimed in claim 1, characterized in that a circulation stream is drawn off from the evaporation space of the high-pressure column top condenser and compressed in a circulation compressor which takes the form of a cold compressor, and the compressed circulation stream is cooled in the main heat exchanger and fed to the high-pressure column.
 13. The method as claimed in claim 1, characterized in that a circulation stream is drawn off from the upper region of the low-pressure column and compressed in a circulation compressor which takes the form of a cold compressor, and the compressed circulation stream is cooled in the main heat exchanger and fed to the high-pressure column top condenser.
 14. An apparatus for producing compressed nitrogen by low-temperature fractionation of air comprising a distillation column system having a high-pressure column and a low-pressure column, and a high-pressure column top condenser and a low-pressure column top condenser, both in the form of condenser-evaporators, wherein the low-pressure column in the operation of the plant has the lowest operating pressure of all the separating columns in the distillation column system, and comprising a main air compressor for production of a high-pressure overall air stream by compressing feed air to an overall air pressure, wherein the main air compressor is the only external energy-driven gas compressor in the apparatus, means of work-performing decompression a first substream of the high-pressure overall air stream, means of introducing the first substream decompressed to perform work into the distillation column system, a main heat exchanger for cooling a second substream of the high-pressure overall air stream. means of introducing the cold second substream at least partly in the liquid state into the distillation column system, means of introducing gaseous top nitrogen from the high-pressure column into the liquefaction space of the high-pressure column top condenser, means of withdrawing a first liquid nitrogen stream from the liquefaction space of the high-pressure column top condenser, means of introducing a first substream of the first liquid nitrogen stream as reflux liquid into the high-pressure column, means of introducing top nitrogen from the low-pressure column into the liquefaction space of the low-pressure column top condenser, means of withdrawing a second liquid nitrogen stream from the liquefaction space of the low-pressure column top condenser, means of introducing a first substream of the second liquid nitrogen stream as reflux liquid into the low-pressure column, means of forming an internal compression nitrogen stream by a second substream of the first liquid nitrogen stream and/or a second substream of the second liquid nitrogen stream, means of increasing the pressure of the internal compression nitrogen stream in the liquid state to a product pressure higher than the operating pressure of the high-pressure column, means of warming the internal compression nitrogen stream brought to the product pressure in the main heat exchanger, means of drawing off the warmed internal compression nitrogen stream as gaseous compressed nitrogen product stream below the product pressure, means of drawing off at least one oxygen-enriched product gas stream from the distillation column system, means of warming the oxygen-enriched product gas stream in the main heat exchanger, and comprising means of drawing off all oxygen-enriched product gas streams in the gaseous state from the distillation column system, characterized in that the exit of the means for work-performing decompression is flow-connected to the high-pressure column, the main air compressor is designed for the compression of the feed air to an overall air pressure at least 5 bar above the operating pressure of the high-pressure column, and the means of increasing the pressure of the internal compression nitrogen stream in the liquid state are designed for increasing the pressure to a product pressure between 20 and 100 bar.
 15. The apparatus as claimed in claim 14, characterized in that the apparatus does not have any means of drawing off a liquid oxygen-enriched product stream from the distillation column system and more particularly does not have any pump for bringing a liquid oxygen-enriched stream to an elevated pressure. 